Change location of Qt kits JSON file

Currently, we place the file into the config directory of the cmake-tools extension.

1. Are we sure that we're always allowed to write there?
2. Our attempt to locate this config folder will fall flat if VS Code is in portable mode (compare https://code.visualstudio.com/docs/editor/portable)

As an alternative, we could write the JSON file in extensionContext.globalStoragePath and point ms-vscode.cmake-tools to it.